In recent years, a DVD (Digital Versatile Disc) has widely generalized as a disc-like information recording medium capable of recording the information at high density.
Here, the conventional recordable DVD (e.g., DVD-R (DVD-Recordable)), in which an optical beam for information recording is applied on the DVD from the side of a substrate, had typically such a cross-sectional structure that the substrate, a recording layer on or from which the information is recorded or reproduced, a reflecting layer for reflecting the applied optical beam, and a cover layer for protecting the reflecting layer and the recording layer are laminated in this order from the side where the optical beam is applied.
As the recording tracks on which the information is recorded, the grooves formed on the substrate are convex as seen from the incident side of the optical beam (accordingly concave as seen from the substrate itself). Each recording track is composed of the recording layer and the reflecting layer formed on each groove. Since the recording layer itself is usually formed by a spin coat method, the recording layer formed in an area opposed to the groove has a greater thickness than the recording layer formed in an area opposed to the area of DVD other than the groove (e.g., area between two adjacent grooves, which is typically referred to as a land). This is due to the fact that when the recording layer is formed by the spin coat method on the substrate on which the groove and the land are alternately formed, a lot of liquid material serving as the recording layer is gathered within the groove.
In recent years, some researches on providing the DVD with high recording density (the DVD having high recording density is hereinafter referred to as a high density DVD) have been carried out, in which one research involves an attempt for attaining the high recording density by shortening the wavelength of the recording or reproducing optical beam to make a recording pit smaller. In the high density DVD, unlike the conventional DVD, the optical beam for information recording is incident on the high density DVD from the side of the cover layer.
Here, if the conventional DVD having such a cross-sectional structure that the recording track is formed on a convex portion as seen from the incident side of optical beam, and the optical beam is applied on the recording track is directly employed for the high density DVD, the recording layer serving as the recording track is formed at the position opposed to the land on the substrate, so that the shape of the recording track may be convex on the side of the cover layer that is the incident side of optical beam in the cross-sectional structure.
Supposing that the recording layer is formed by the same spin coat method of the conventional DVD, employing this substrate, the groove itself of the substrate can not be formed deeply, if a desired thickness of the recording layer tries to be attained by gathering a lot of liquid material serving as the recording layer at the position opposed to the land. As a result, a lot of liquid material is also gathered in the groove other than the land, whereby the thickness of the recording layer formed as the recording track in the area opposed to the land is equal to or less than the thickness of the recording layer formed in the area opposed to the groove.
However, if a recording optical beam having a shorter wavelength is directed to the high density DVD formed in this way, not only the recording layer in the area opposed to the land but also the (thicker) recording layer in the area opposed to the groove are provided with the characteristics as the recording pits due to irradiation, resulting in a problem that a recording interference phenomenon occurs that the pseudo recording pits are formed in the area other than the recording track on the land.
Due to this problem, when the recorded information is reproduced, the information from the recording pits other than the recording pits for carrying the information to be essentially reproduced is intermingled into the reflected light of the optical beam for information reproduction, aggravating the C/N ratio (ratio of carrier wave level to noise level) at the time of reproduction, and leading to another problem that the correct information reproduction is not performed.